/**
 * Created by fengxingck on 2017/5/3.
 */
import java.util.StringTokenizer;
   import java.util.Stack;
           public class MyDC
  {
             /** constant for addition symbol */
             private final char ADD = '+';
           /** constant for subtraction symbol */
           private final char SUBTRACT = '-';
           /** constant for multiplication symbol */
           private final char MULTIPLY = '*';
           /** constant for division symbol */
           private final char DIVIDE = '/';
           /** the stack */
           private Stack<Integer> stack;

           public MyDC() {
             stack = new Stack<Integer>();
           }

           public int evaluate (String expr)
           {
             int op1, op2, result = 0;
             String token;
             StringTokenizer tokenizer = new StringTokenizer (expr);

             while (tokenizer.hasMoreTokens())
             {
               token = tokenizer.nextToken();

               //如果是运算符，调用isOperator
               if (isOperator(token) == true)
               {
                   int a = stack.pop();//从栈中弹出操作数2
                   int b = stack.pop();//从栈中弹出操作数1
                   result = evalSingleOp(token.charAt(0), a, b); //根据运算符和两个操作数调用evalSingleOp计算result;
                   stack.push(result); //计算result入栈;
               }
               else//如果是操作数
               stack.push(Integer.parseInt(token));//操作数入栈;
             }

             return result;
           }

           private boolean isOperator (String token)
           {
             return ( token.equals("+") || token.equals("-") ||
                      token.equals("*") || token.equals("/") );
           }

           private int evalSingleOp (char operation, int op1, int op2)
           {
             int result = 0;

             switch (operation)
                    {
               case ADD:
                 result = op1 + op2;
                 break;
               case SUBTRACT:
                 result = op1 - op2;
                 break;
               case MULTIPLY:
                 result = op1 * op2;
                 break;
               case DIVIDE:
                 result = op1 / op2;
             }

             return result;
           }
  }